Ah... Some kind of key. Not all world travelers have it, but the important ones usually do. I'd explain, but it'd probably go over your head.

[and it's not like josh cared that much about the legends of keyblade wielders when he was composer, so he doesn't want to talk about that too much. he turns to look at Neku suddenly, his expression an odd mix of earnest (for once) curiosity and reluctance]

People can travel to worlds. Did you ever wonder what other things could, too?